Trona ore is a naturally occurring mineral that consists primarily of sodium sesquicarbonate (Na2CO3—NaHCO3-2H2O) and about 4 to 12 percent insoluble materials consisting mainly of shale. In the vicinity of Green River, Wyo., trona ore deposits are found at depths ranging form about 800 to 1800 feet underground. The main trona bed varies from 8 to 18 feet in thickness and other beds of less thickness separated by layers of shale are usually found above the main trona bed. Although some solution mining techniques are now being used, trona ore is frequently mechanically mined and carried to the surface for further processing.
Various processes for the production of sodium carbonate from sodium sesquicarbonate are known. In the “monohydrate process,” trona ore is crushed, calcined to convert sodium bicarbonate values to sodium carbonate, and dissolved; the solution is filtered, treated with activated carbon to remove soluble organic compounds, and evaporated to crystallize sodium carbonate monohydrate. The monohydrate is dried to produce anhydrous sodium carbonate. See, for example, U.S. Pat. No. 2,962,348, which describes a typical monohydrate process.
When crude trona is crushed, a broad distribution of particle sizes is obtained. Typical sizing, expressed throughout in terms of U.S. Standard Sieves, illustrate this. As discussed in U.S. Pat. No. 3,869,538, ore crushed to 100%−⅜ inch mesh is typically 0%+⅜ inch, 25%+4 mesh, 80%+100 mesh, and 90%+270 mesh. At 100%−¼ inch mesh, a typical distribution of 0%+¼ inch, 10%+4 mesh, 50%+16 mesh, 75%+100 mesh, and 90%+325 mesh. Ore crushed to pass 8 mesh 100% will typically be 0%+8 mesh, 20%+20 mesh, 35%+40 mesh, 50%+100 mesh, 65%+200 mesh, and 80%+400 mesh. Using the entire product from a crushing circuit provides more efficient use of the raw material than if the product is separated into fractions of narrower range.
The smallest particles of trona formed when crude trona is crushed are referred to as trona dust. Handling of trona dust is problematic for a number of reasons including environmental, health and maintenance hazards due to the low density of these small particles. For example, containment of trona dust is difficult when it is transferred from the crusher and loaded onto the calciner feed belt. Dust is kicked up during this process, resulting in inefficient transfer of the trona dust to the calciner and unnecessary exposure to the trona dust. Trona dust frequently has to be cleaned from the equipment and processing unit.
In addition, processing of trona dust in conjunction with granular trona decreases the efficiency of the calciner as compared to processing granular trona alone. Processing trona dust in a rotary kiln-type calciner increases slag, causes refractory brick damage, increases formation of soluble silica and organics, and reduces the feed end temperature of the calciner thereby reducing heat transfer to larger ore size particles. Because there is a temperature gradient in a rotary kiln-type calciner, over calcination occurs in the hot end of the calciner, especially near the surface of particles, which become overexposed to the hot gases. Fine particles collected from the calciner therefore contain a higher level of soluble organic compounds than the coarse particles in the product.
U.S. Pat. No. 3,869,538 describes a method of calcining crushed trona of broad size range to crude sodium carbonate in a fluid bed calciner at 125-225° C. A more uniform temperature in the fluid bed calciner results in fine particles not becoming over calcined, and no concomitant increase in formation of soluble organic material is obtained. Soluble organic materials are but one problem of processing trona dust.
In addition to the trona processing industry, dust processing and recycling is important the cement, alumina, clay and lime industries, for example. U.S. Pat. Nos. 5,007,823 and 6,241,514 describes methods and apparatus, respectively for dust recycling, particularly with application to cement dust. In both of these patents, the dust is recycled back to the kiln in combination with an enriched oxygen atmosphere to facilitate processing of the dust. What is needed are improved methods which address all of the previously discussed issues for handling, processing, and recycling dust, in particular, trona dust to form sodium carbonate.